Revert "powerpc/tm: Abort syscalls in active transactions"

[karo-tx-linux.git] / Documentation / powerpc / transactional_memory.txt

diff --git a/Documentation/powerpc/transactional_memory.txt b/Documentation/powerpc/transactional_memory.txt
index ba0a2a4a54ba1ffcb484786381b91f5113a62ad6..ded69794a5c09da762db7c4c61cc3d23e619d0fa 100644 (file)
--- a/Documentation/powerpc/transactional_memory.txt
+++ b/Documentation/powerpc/transactional_memory.txt
@@ -74,23 +74,22 @@ Causes of transaction aborts
 Syscalls
 ========
 
-Syscalls made from within an active transaction will not be performed and the
-transaction will be doomed by the kernel with the failure code TM_CAUSE_SYSCALL
-| TM_CAUSE_PERSISTENT.
+Performing syscalls from within transaction is not recommended, and can lead
+to unpredictable results.
 
-Syscalls made from within a suspended transaction are performed as normal and
-the transaction is not explicitly doomed by the kernel.  However, what the
-kernel does to perform the syscall may result in the transaction being doomed
-by the hardware.  The syscall is performed in suspended mode so any side
-effects will be persistent, independent of transaction success or failure.  No
-guarantees are provided by the kernel about which syscalls will affect
-transaction success.
+Syscalls do not by design abort transactions, but beware: The kernel code will
+not be running in transactional state.  The effect of syscalls will always
+remain visible, but depending on the call they may abort your transaction as a
+side-effect, read soon-to-be-aborted transactional data that should not remain
+invisible, etc.  If you constantly retry a transaction that constantly aborts
+itself by calling a syscall, you'll have a livelock & make no progress.
 
-Care must be taken when relying on syscalls to abort during active transactions
-if the calls are made via a library.  Libraries may cache values (which may
-give the appearance of success) or perform operations that cause transaction
-failure before entering the kernel (which may produce different failure codes).
-Examples are glibc's getpid() and lazy symbol resolution.
+Simple syscalls (e.g. sigprocmask()) "could" be OK.  Even things like write()
+from, say, printf() should be OK as long as the kernel does not access any
+memory that was accessed transactionally.
+
+Consider any syscalls that happen to work as debug-only -- not recommended for
+production use.  Best to queue them up till after the transaction is over.
 
 
 Signals
@@ -177,7 +176,8 @@ kernel aborted a transaction:
  TM_CAUSE_RESCHED       Thread was rescheduled.
  TM_CAUSE_TLBI          Software TLB invalid.
  TM_CAUSE_FAC_UNAV      FP/VEC/VSX unavailable trap.
- TM_CAUSE_SYSCALL       Syscall from active transaction.
+ TM_CAUSE_SYSCALL       Currently unused; future syscalls that must abort
+                        transactions for consistency will use this.
  TM_CAUSE_SIGNAL        Signal delivered.
  TM_CAUSE_MISC          Currently unused.
  TM_CAUSE_ALIGNMENT     Alignment fault.